This invention relates to dynamoelectric machines and more particularly to such machines which include permanent magnets assembled in a laminated pole rotor.
Available permanent magnets, such as rare earth magnets, have made it attractive to construct dynamoelectric machine rotors with permanent magnets rather than with wound coils for providing field excitation. Key advantages of the permanent magnet rotors such as reduction in weight and size, are particularly important in applications such as aircraft.
U.S. Pat. No. 4,354,126, which is hereby incorporated by reference, describes a dynamoelectric machine having a permanent magnet rotor which offers high strength construction, a laminated ferrous pole structure to obtain low eddy current losses, circumferential orientation of the rotor magnets, complete containment of the magnets, and an economically manufacturable structure. Such a rotor is especially suited to the high energy product characteristics of rare earth magnets. The magnetic structure of that rotor essentially comprises high strength, non-magnetic structural laminations having slots for the permanent magnets, alternating with magnetic laminations which have sufficient structure to conduct the magnetic flux to the machine air gap. The rotor ends are closed with non-magnetic laminations to close the magnet slots and wedges are used to contain the permanent magnets.
Although the rotor structure disclosed in U.S. Pat. No. 4,354,126 has proven to exhibit high strength characteristics at a peripheral speed of 471 feet per second, higher speed designs are still desirable. For a given speed, rotors having many poles, such as 12 pole rotors, will generate a frequency three times greater than a 4 pole design when operated as a generator. For extreme speeds, the high frequency of the many pole design is undesirable. In addition, when operating as a motor, a driving frequency for a 12 pole machine is three times that of a four pole machine. This imposes extremely fast switching speed requirements on an inverter drive. It is therefore desirable to use machines with a smaller number of poles for extremely high speed applications.
When one attempts to extend the design of the rotor disclosed in U.S. Pat. No. 4,354,126 to a rotor having a small number of poles, it becomes apparent that the ratio of the radial depth to the circumferential length available for the permanent magnets is much smaller. Therefore, the magnetic flux cannot be concentrated as well at the air gap. Furthermore, the circumferential length available to the magnet is much more than is needed. This invention overcomes these difficulties by providing a high speed laminated permanent magnet rotor which is suitable for a low pole number design.